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Marconiphone 296

296 AC

Manufactured in 1934, this was a classic set of the 1930's and was made to a high specification. The valves are standard 4V heater types on B4,B5 and B7 bases. It was one of the first generation of domestic superhet receivers to use a specially designed multi-electrode combined local oscillator/mixer valve, in this case an MX40 heptode.

Marconiphone 296 This is a multiplicative mixer. (Earlier circuits used additive mixing, which relies on the non-linear characteristic of the mixing valve to produce the IF difference signal - see the technical notes on this site.)

A shadow or "fluid light" type of tuning indicator is used. It works with a mechanical moving vane (driven by a meter movement in the anode circuit of the variable-mu IF valve) to cast a shadow on a backlit scale. The valve line-up is:

MX40 Frequency changer
VMS4B IF amplifier (IF = 125 kHz)
MHD4 Detector and AF amplifier
PX4 Output
U12 Mains rectifer

The HT is smoothed using the field coil of the mains energised loudspeaker in the HT negative return. Since bias for the output valve is derived from the voltage drop across the coil, this results in the can of one of the smoothing electrolytics not being at chassis potential. Insulating washers are used to fix it in place.

The band pass tuning arrangements are complex because the low IF makes it difficult to achieve image rejection without high selectivity here. The vanes of the oscillator section of the ganged tuning capacitor are specially shaped, so no oscillator padder is needed. Delayed AVC is employed.

Considerable care has been taken with the design to ensure a high quality audio output. The volume control circuit applies bass boost at low volume. The first AF valve is transformer coupled (parallel feed) to the PX4, and a tone compensation inductor incorporated. A separate tone control is provided. A "hum-bucking" control is used in the filament circuit of the directly heated PX4 output valve.

Interstation noise suppression (obtained by over-biasing the IF valve) is switched on and off by pulling out the volume control knob. A sensitivity control for this is provided at the back of the set.

The original price was Ł14 3s. 6d.

The circuit is futher discussed in the Technical note on this site (page 9).

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